1. FIELD OF THE INVENTION
This invention relates to .gamma.-ray measurement and in particular to .gamma.-ray measurement utilizing multiple Compton scattering.
In this specification .gamma.-ray means high energy photons and includes soft and hard X-rays. Specifically the object of this invention is photons having an energy from about 100 keV to about 5 MeV.
Further, a multiple Compton scattering, called hereinbelow reactions, includes both a phenomenon, by which a photon is scattered a plurality of times by the Compton effect and a phenomenon, by which it disappears at last while giving an electron its energy by the photoelectric effect.
2. DESCRIPTION OF THE RELATED ART
.gamma.-ray is measured in various fields. Taking computer tomography (CT) as an example, there are roughly known two types, i.e. positron CT and single gamma CT. In the positron computer tomography (CT) a radioactive isotope emitting positrons is introduced in a human body, an animal or a plant and oppositely directed 2 photons, each of about 511 keV, generated at the pair annihilation of a positron and an electron are measured by the coincidence method by means of a number of .gamma.-ray detectors arranged suitably, such as NaI, CsI, bismuth germanate (Bi.sub.4 Ge.sub.3 O.sub.12, BGO), etc. In this way the spatial distribution of .gamma.-ray sources, i.e. the isotope, is measured. The location of a radiation source is confined to a straight line connecting two detectors, which have detected the two photons by the coincidence method. Further, the position or distribution of radiation sources of a same nature is measured 3-dimensionally by detecting a number of pairs of photons and treating statistically detected data by means of a computer.
Single gamma CT uses a shield. When a gamma ray is detected, a gamma ray source is estimated to be located on a line connecting a detector and a shield. A multiplicity of gamma rays from the same source are detected and such detected data are statistically processed in a computer to measure the position or distribution of gamma ray sources three-dimensionally.
In order to measure the position of incidence of the .gamma.-ray, an assembly of a number of unit detectors is prepared and it is sufficient to examine which unit detector the .gamma.-ray enters. The position of incidence can be detected, e.g. by forming diodes in a sheet shaped semiconductor substrate, forming a number of strip-shaped electrodes on the front and backside surfaces thereof, which cross perpendicularly to each other, and detecting between which electrodes electric current flows.
In order to know in which direction the .gamma.-ray enters by means of a measuring device, single Compton method can be employed or another method can be employed by which a collimator is located in front of a .gamma.-ray detector so that only .gamma.-ray entering in a predetermined direction is detected.
A single Compton method is known for relatively low energy .gamma.-ray, by which the incident direction or the polarization of the incident .gamma.-ray is measured by using a single Compton scattering and another reaction. The direction on the polarization of X-ray coming from e.g. the universe is measured by means of a measuring device consisting of a position sensitive radiation detector disposed in the front portion, closer to the object to be measured, measuring the position of the Compton scattering and the energy of a recoil electron and an NaI scintillation counter disposed behind the detector by a suitable distance, which absorbs the X-ray or the .gamma.-ray after the scattering, giving rise to scintillation, in order to measure its energy and position.
In the positron computer tomography (positron CT) the energy of the incident .gamma.-ray is about 510 keV, which is sufficient to give rise to a plurality of Compton scatterings, i.e. which is an energy sufficient for every incident .gamma.-ray to produce a plurality of detection signals within a detecting device.
In single gamma ray tomography, gamma rays ranging from several hundreds KeV to several MeV are used. Such gamma rays also have sufficient energy to produce a plurality of Compton scattering in a detector. The positron CT had the following restrictions.
(a) Since radioactive isotopes usable therefor are limited to nuclides, which emit positrons and have relatively short half-lives, it can be utilized only at a location, in the neighborhood of which such nuclides can be produced. PA1 (b) The positional precision for defining the position of a radiation source is determined by the positional detecting precision of the .gamma.-ray detecting device. A several millimeter square is the lower limit in practice at present even by using a collimator, etc. The use of the collimator brings about lowering of the counting efficiency. PA1 (c) Because of the coincidence measurement the counting efficiency is very low and therefore a measurement takes a long time. PA1 (d) The case where a radioactive isotope emitting positrons is used, since a positron is emitted with a relatively high energy, the position, where it annihilates to produce two photons, is apart from that of the radiation source. PA1 (a) In order to increase the counting efficiency, with which a photon induces a reaction within a detector as expected, it is necessary to increase the thickness of the sheet-shaped .gamma.-ray detector, within which it is expected for the photon to induce a Compton scattering. On the contrary, in order to measure the positions of two successive reactions with a high precision and to increase the precision of the measurement of the direction of the incident .gamma.-ray, it is necessary to reduce the thickness of the sheet-shaped detector. Since the detection efficiency is lowered when the thickness of the detector is reduced, the detection efficiency and the measurement precision are in a contradictory relation. PA1 (b) When the energy of the .gamma.-ray, which is to be measured, is above several keV, the number of reactions until a photon is finally absorbed is increased and the result, it is not possible to determine the direction of the .gamma.-ray after a scattering, which is one of the most important factors for determining the direction of the incident .gamma.-ray, with a high precision and therefore the measurement precision for the direction of the radiation and the position of the source is lowered. PA1 (c) In order to increase the detection efficiency, the detector located behind the examined body should be predominantly thicker than the detector located in front thereof, within which it is expected for the first Compton scattering to be induced. In this case the probability is also increased, that the .gamma.-ray passes through the front detector without reaction, produces a back scattering by the Compton effect in the proximity of 180.degree. within the back detector, and is detected by the front detector. If the data thus obtained were interpreted, supposing that the first scattering is produced within the front detector and the second scattering is induced in the back detector, this gives rise to a sort of noise, which lowers the reliability of the measurement.
The method for measuring .gamma.-ray the single gamma CT utilizing by the single Compton scattering method has the following problems.
For these reasons the single Compton method is not practical for the energies above several keV. On the other hand the single gamma CT method using a collimator lowers extremely the counting efficiency, because the direction of the radiation is restricted by the fine collimator (made of e.g. lead). Further it is not practical, unless the distance between the .gamma.-ray source and the detector is fixed in a certain extent and stereoscopic observation is impossible with a single